This invention relates to article inspection systems and, particularly, to vision systems for inspecting work pieces such as plastic molded closure caps for containers and the like.
During the manufacture of many parts such as plastic molded closure caps, a number of defects in the closure cap may exist which should cause the cap to be rejected. Commonly, closure caps of this type have a liner inserted therein against the inner surface of an end wall of the cap. Typically, the cap has a skirt projecting annularly from the peripheral rim of the end wall and the skirt may include a closure or sealing mechanism such as threads for cooperation with mating threads around the neck of a bottle, container or the like. Examples of defects in such closure caps include a liner which is positioned off center within the closure, a missing liner, a malformed liner (commonly referred to as a xe2x80x9cmoon-cutxe2x80x9d liner), a cap which is asymmetric or off-round, a cap having an edge broken or flashing on the edge from extraneous plastic material, a pull tab defect on the liner or other similar problems. Such flaws or defects are sometimes produced during the manufacturing process. and/or as a result of contamination or damage after manufacture, but prior to the filling of the container.
Machine vision systems represent one technology for acquiring or sensing an image of at least a selected portion of a work piece, such as a cap as previously described, through an electronic sensor or camera. The image generated by the camera is then analyzed by a computer program for one or more of the above-described defects. Vision systems are commonly used to determine the existence of any marks or defects in the image of the cap and the acceptability of any such marks or defects by use of a vision computer as described.
While human vision may out perform its automatic equivalent in the ability to analyze very complex, everyday scenes, when it comes to repeated tasks, such as the inspection of plastic molded caps over and over again, a human observer understandably tires, loses concentration and makes mistakes. Machine vision inspection of such articles is known to provide some important advantages, including sophisticated image processing/analysis, repeatable performance, image acquisition for diagnosis and set up, ability to inspect a variety of articles in large tolerance and required part placement. Moreover, at inspection rates of up to 1600 parts per minute or more, each part or cap spends on the order of 33 milliseconds at an inspection station. At such speeds, only a machine vision system is fast enough to reliably and repeatedly inspect such articles.
While known vision systems have the above-described advantages for inspecting articles such as plastic molded caps and the like, they do have specific and significant limitations. Vision systems typically rely on television or video cameras to image the article to be inspected and detect any flaws. The resolution of the camera, or its ability to detect a flaw, is directly related to its ability to capture an accurate and reliable image of each individual cap, article or similar item. Typically, plastic molded caps, for example, are manufactured by the tens of thousands and each individual cap must be inspected by the vision system for quality control purposes. The large volume of caps are typically gathered in an accumulated mass and, at best, are similarly oriented on a flat surface. For accurate vision inspection and detection of flaws, the vision system must be able to precisely and accurately produce an image of each individual cap without interference from the surrounding environment or other caps.
Furthermore, inspection rates required of such systems mandate that the individual images be serially produced, analyzed and acted upon accordingly for each individual cap, once again without interference, for accurate detection of relatively small flaws or problems.
Additionally, plastic molded caps are commonly made in the wide range of colors and lining materials depending on a particular manufacturer""s packaging requirements, marketing scheme or other such demands. As such, industrial vision systems should be capable of inspecting any and all such cap colors with equal accuracy, precision, efficiency and speed. However, because of the limitations in known vision systems with respect to the video cameras and lighting requirements, specific colors, such as white caps with white or foil liners or other light colors, are not accurately inspected for all possible defects. Specifically, known vision systems require a visual contrast between the cap and the liner to accurately inspect for the presence of defects such as off-center, missing or moon-cut liners within the cap.
Another problem with known vision systems involves the presence of foil or similar liners in the caps that tend to reflect light. Commonly, the caps and liners are illuminated or lighted from above to produce an image for inspection. However, foil liners and other reflective materials produce a glare when the light impinges thereon. The glare significantly reduces the clarity of the image of the cap and liner being inspected and thereby significantly reduces the accuracy and reliability of the inspection system.
The present invention provides a machine vision inspection system for inspecting work pieces such as caps and other articles and an associated method for doing so which overcomes the above-stated and other limitations with known systems/methods.
In a presently preferred embodiment, this invention is an inspection system for inspecting each of a series of serially fed work pieces in a stream of work pieces, such as plastic molded caps or the like. The system includes a feed conveyor to serially feed the caps or work pieces, each of which is typically in contact with adjacent work pieces on the feed conveyor in an accumulated mass or the like. The feed conveyor advances the caps to an inspection ramp which in a presently preferred embodiment is inclined between 35xc2x0 and 50xc2x0, and most preferably at 40xc2x0 with respect to a horizontal plane. The inspection ramp has a reduced friction upper surface upon which the caps or other articles advance downwardly from a top end of the inspection ramp toward a bottom end. An optional discharge conveyor is located at the bottom end of the inspection ramp to receive and discharge each of the caps for collection, packaging and/or further processing.
Advantageously, the inspection ramp is inclined so that as the caps which are in contact with one another and therefore difficult for a vision system to accurately inspect and discriminate at a top end of the ramp advance by gravity along the reduced friction surface through an inspection station located between the top and bottom ends of the ramp. The incline of the ramp produces a separation distance between each of the caps so that each cap can be individually and accurately inspected at the inspection station for defects or the like. Preferably, a pair of spaced guide rails are positioned on the lateral sides of the caps to provide for accurate lateral positioning of the caps with respect to the inspection station on the ramp.
The inspection station in a presently preferred embodiment includes an inspection window in the ramp, an infrared or other color LED strobe light source and a camera. The light source is preferably located on a back side of the inspection ramp to project light through the inspection window to back-light and illuminate each of the caps as they pass above the window. Back-lighting of the caps avoids the above-described problem of glare from foil caps and likewise offers a contrasting image even with white caps and white liners for accurate imaging. The inspection window and light source are preferably aligned with the camera which is located on a top side of the inspection ramp and oriented generally perpendicularly with respect to the inspection ramp at the inspection station. Preferably, the positioning of the light source, inspection window and camera provides for a full and complete image of the cap for accurate resolution and detection of possible defects in the cap.
The system also includes a processing unit such as a computer or the like operably coupled to the camera to analyze the images of the caps generated by the camera with respect to predetermined quality control standards. For example, specific defects as described hereinabove, if detected by the camera, are outside of the predetermined quality control standards utilized by the computer to analyze each of the images generated by the camera. If a particular cap fails the predetermined quality control standards, a rejection mechanism, typically an air jet or the like, is coupled to the processing unit to receive a control signal from the. processing unit and, if such a signal is received by the air jet or other rejection mechanism, the identified cap is then removed from the stream by the air jet or other rejection mechanism. The rejected cap is then discarded, analyzed or recycled as is appropriate.
This invention in a presently preferred embodiment overcomes the above-described disadvantages of known vision inspection systems by accurately and reliably providing a separation distance between each of the serially fed work pieces, articles, caps or the like to be inspected as a result of the inspection ramp and the inclination thereof. Further, the processing or inspection rate is not diminished as a result of the orientation and configuration of the inspection ramp and inspection station components thereby providing inspection rates as high as 1600 per minute or more depending upon the size of the items being inspected and other operational requirements.
Furthermore, heretofore difficult to inspect or analyze materials such as white and other light colored plastic molded caps and liners are reliably and effectively illuminated for accurate imaging and detection by the camera and processing unit because of the opportunity for back lighting the cap with an infrared or other color LED strobe light source.